Title 24 Requirements for Process Boilers
Three requirements at three different sizes: combustion air positive shutoff from 2.5 MMBtu/h, variable speed drives on combustion air fan motors of 10 hp and above, and excess oxygen control between 5 and 10 MMBtu/h.
A process boiler is not a space heating boiler
The distinction matters before any threshold does. A process boiler raises steam or hot water for something the business does — sterilising, cooking, cleaning, driving a manufacturing step. A boiler heating the building is a space conditioning system and is assessed under the mechanical requirements instead.
The same piece of equipment can be either, and occasionally both. What decides it is the load it serves, not the boiler's specification.
2.5 MMBtu/h: combustion air positive shutoff
Combustion air positive shutoff is required on process boilers with an input capacity of 2.5 million Btu per hour and above — but only where the boiler is designed to operate with a nonpositive vent static pressure.
Both conditions are required, and the second is the one that gets missed. In a naturally drafting boiler the stack keeps pulling air through the firebox after the burner stops, carrying the heat you just paid for straight outside. A positive shutoff closes that path. A boiler that runs with positive vent pressure does not have the problem and is not caught.
10 hp: variable speed on the combustion air fan
Combustion air fans with motors of 10 horsepower or larger must be driven by a variable speed drive — or, alternatively, include controls that limit motor demand.
The alternative is written in deliberately, and it is worth reading before a drive is specified. Where a fan genuinely runs at one duty most of its life, demand-limiting controls may satisfy the requirement more sensibly than a drive that never modulates.
5 to 10 MMBtu/h: excess oxygen control
Newly installed process boilers with an input capacity from 5 to 10 MMBtu/h must hold excess stack-gas oxygen at 5.0 per cent or less by volume on a dry basis, across firing rates from 20 to 100 per cent.
Read the range carefully — it is a band, not a floor. The requirement applies to boilers between 5 and 10 million Btu per hour, and the performance has to hold across almost the whole firing range, not merely at full fire where a burner is easiest to tune.
Every percentage point of excess air is air heated to stack temperature and thrown away. Five per cent oxygen is a demanding but achievable target, and holding it down to 20 per cent firing is what requires oxygen trim rather than a one-time commissioning tune.
Why the three thresholds do not line up
They look arbitrary next to each other, and they are not. Each requirement is set where its own measure stops paying: positive shutoff earns its cost on any reasonably sized drafting boiler, variable speed needs a motor big enough to be worth a drive, and oxygen trim needs a boiler that modulates enough for trim to have something to do. A boiler can easily engage one and not the others.
Establish this at equipment selection
Input capacity in MMBtu/h, vent static pressure design, and combustion air fan motor size. Those three numbers decide which of the requirements apply — and all three are fixed by the equipment order, not by anything that can be adjusted on site afterwards.